I would take a different approach. I wouldn't use locators to add more particles to the simulation; rather I would use a [`ClickPane`](http://reference.wolfram.com/mathematica/ref/ClickPane.html). Nor would I use a slider to generate update events; rather I would use a [`Button`](http://reference.wolfram.com/mathematica/ref/Button.html)

    Manipulate[
      If[update, update = False; pts = updater[]];
      ClickPane[
        Framed@Graphics[{Blue, PointSize[0.02], Point[pts]}, PlotRange -> {{-5, 5}, {-5, 5}}], 
        AppendTo[pts, #] &],
      {{update, False}, ControlType -> None},
      {{pts, {{0, 0}}}, ControlType -> None},
      Button["Update", update = True],
      Initialization :> (
        vels[n_] := RandomReal[{-1, 1}, {n, 2}];
        updater[] := (pts = MapThread[(#1 + #2) &, {pts, vels[Length@pts]}])
      )]

![manipulate][1]

Give it a try and see if it satisfies your needs.

  [1]: https://i.sstatic.net/H3xiV.png